Re: RFC: Code size improvement for global alloc

[Vladimir Makarov <vmakarov at redhat dot com>]

Daniel Jacobowitz wrote:

> Testing: I tested this patch with bootstrap / make check (all languages
> except Ada) on x86_64-pc-linux-gnu.  I also tested an equivalent patch for
> arm-none-elf using csl-arm-branch.  There were no regressions in either
> case.  Bootstrap got 5.5 seconds faster out of 81 minutes, which is in the
> noise.  On x86_64 there was a size improvement of 269 bytes out 590K in some
> random files from cc1-i-files; on ARM there was a size improvement of about
> 0.05% in CSiBE.
>
> OK?  Comments?
>
>  
I think that at least you need to introduce a new flag for this.  You 
can not make it by default until you prove that there is a performance 
improvement on a credible benchmark (better for SPEC95 or SPEC2000) on 
major platforms (x86, x86-64, ppc).  Although the optimization removes 
move insns, the final result might be worse after the reload because the 
reload might expel the coalesced pseudo-registers from a hard register 
and only one pseudo-register when coalescing did not happen.

Also I have a code for more common form of coalescing in global (please 
see gcc summit proceeding) which coalesces all registers not only global 
and local ones (as in your patch) and trying the two hard registers (not 
only one of the global).  It even coalesces two registers one of which 
or the both ones got memory if it is profitable.  It also coalesces 
pseudo-register according to the frequencies of the move insns to get a 
better results.

Besides features mentioned above I see the following possible 
improvements in your patch:

 1. trying hard registers from alternative register class too.
 2. trying call used hard registers if it is profitable.

Still my patch (and yours) is a constrained form of coalescing because 
it is done after the register allocation (assigning).  More common form 
of coalescing would require an iterative approach to the register 
allocation (and implementing register live range spilling to undo 
coalescing if it is necessary). It is one more way to improve the 
register allocation.

I am going to submit my patch in a few weeks.  It would be interesting 
to compare your patch with mine.  I will probably do that.

Vlad